This project is designed to develop radiobiological tests both in vitro and in vivo for the quantitative measurement of the toxicities of low energy (including Auger) electrons and alpha particles (especially astatine-211) produced by radioactive decay and to use these systems to determine the radiotoxicity of radionuclides commonly used in nuclear medicine, as well as other nuclides of current scientific interest. It also intends to develop a strategy for treating tumors with administered radioactive materials using iodine-125 labeled 5-iododeoxyuridine as the prototype for such therapy.